1. Field of the Invention
The present invention relates to an interface apparatus for data communications between a high-power device and a low-power device, and a data communications system using the interface apparatus.
2. Description of the Related Art
U.S. Pat. No. 5,504,864 discloses the structure for data communications between a high-power device such as a desktop computer (desktop PC) and a low-power device such as a hand-held computer (hand-held PC) via pins 2 and 3 in RS-232C connectors.
FIG. 4 illustrates connecting a desktop PC 102 to a hand-held PC 104 via a communications interface apparatus 100 having a cable 110 and connectors 112, 114 in accordance with a data transmitting system disclosed in U.S. Pat. No. 5,504,864. Reference numerals 106 and 108 respectively denote serial ports of the desktop PC 102 and the hand-held PC 104. A level conversion circuit 116 is housed in the connector 114 that enables voltage level conversion between the desktop PC 102 and the hand-held PC 104.
FIG. 5 is a circuit diagram of the communications interface apparatus 100. To transfer data from the desktop PC 102 to the hand-held PC 104, the level conversion circuit 116 includes an NPN transistor Q1 with its base being connected to a TxD pin of the connector 112 through a resistor R1 as well as to a ground pin GND of the connector 112 through a resistor R2.
The collector of the transistor Q1 is connected to a receive contact RX on the connector 114. The emitter of the transistor Q1 is connected to the ground GND on the connector 112 through a resistor R3 and is directly connected to a ground pin GND on the connector 114.
To transfer data from the hand-held PC 104 to the desktop PC 102, the level conversion circuit 116 comprises a field-effect transistor (FET) Q2, a zener diode D1 and a resistor R5. A receive contact RxD of the connector 112 is connected to a negative voltage source RTS (-12 volts) through the resistor. The gate of the FET Q2 is connected to a transmit contact TX of the connector 114 through a resistor R6 and to the ground pin GND on the connector 112 through a zener diode D2.
The anode of the diode D1 is directly connected to the ground pin GND of the connector 112, and to the ground pin GND of the connector 114 through the resistor R3. The cathode of the diode D2 is connected to the gate of the transistor Q2. The drain of the transistor Q2 is connected to a receive contact RxD of the connector 112 through a resistor R7.
A level converting operation for data transmission from the desktop PC 102 to the hand-held PC 104 via the cable 110 will now be described.
The receive contact RX on the connector 114 is kept high (5 volts) internally by the hand-held PC 104. The TxD line of the connector 112 is low and the transistor Q1 is in an off state when the desktop PC 102 is sending a logical zero (-12 volts). In this instance, transistor Q1 is an "open-collector," and thus has no effect on the receive contact RX of the connector 114, that is, the voltage at the receive contact RX is kept at 5 volts.
Conversely, when a logical one (+12 volts) is being transmitted from the desktop PC 102, the TxD line becomes high and turning transistor Q1 "on", thus, pulling the receive contact RX on the connector 112 to the ground GND (0 volt).
The level conversion for data transmission from the hand-held PC 104 to the desktop PC 102 via the cable 110 will now be described.
The diode D1 sets the threshold for turning Q2 on and off by clamping he positive voltage source DTR (+12 volts) of the connector 112 to approximately 5.1 volts. The potential at the gate of the FET Q2 will be around 3-5 volts, and the FET Q2 will be in an off state when the transmit contact TX of the connector 114 is high (+5 volts). Thus, there is no current flowing between the source and drain, and the receive contact RxD of the connector 112 will continue to be pulled low (-12 volts) by the negative voltage source RTS (-12 volts) of the connector 112.
When the transmit contact TX of the connector 114 goes low (0 volt), the FET Q2 turns on, and the receive contact RxD of the connector 112 is connected to the clamped positive voltage source DTR (+12 volts), pulling RxD to a high state.
The level conversion circuit of the above conventional communications interface apparatus, however, has a large circuitry because it requires the zener diodes and the like. The circuit is powered by the high-power device, therefore, those diodes are necessary for pulling (clamping) the supplied relatively high voltage to a predetermined low voltage corresponding to the low-power device by lowering the threshold for switching the gate of the FET. The clamping by the diodes is necessary for correct switching of the FET in accordance with a signal from the low-power device.
Moreover, the cable needs a large connector because the level conversion circuit is housed in the connector. Such the a cable, moreover, has a high manufacturing cost and is inconvenient for handling.